481 



Order RODENTIA. 



1758. Olires Linnaeus, Syst. Not. I, 10th ed., p. 56 (part). 



182Y. Rodentia Griffith, Cuvier's Animal Kingdom, Mamm., in, p. 61. 



Geographical distribution. — Continents and larger islands of 

 the entire world, New Zealand and the Antarctic excepted.* 



Characters. — Terrestrial (occasionally semi-aquatic) placental 

 mammals, incapable of true flight (a parachute-like membrane 

 present in certain groups) ; feet unguiculate ; dentition consisting 

 of izi functional scalpriform incisors (a second pair of minute, 

 terete, non-trenchant incisors present in upper jaw in the 

 Duplicidentata) growing from a persistent pulp, and a row of 

 from two to six tuberculate or laminate cheek-teeth, the crowns 

 of which show no distinct traces of tuberculo-sectorial structure ; 

 canines absent ; a wide diastema between incisors and cheek- 

 teeth. 



BemarJcs. — The members of the order Rodentia are imme- 

 diately recognizable by the peculiarities of their dentition, no 

 near approach to which is found in any other group of living 

 mammals. They are all relatively small animals, the largest 

 member of the group, the South American Hydrochoerus hydro- 

 chceris, attaining a weight of only about 50 kg., while the largest 

 European species, the beaver, is less than half this size. In 

 structure they show variety exceeded by the Insectivora only, 

 while in number of genera and species they far surpass all other 

 groups of mammals. About thirty families are recognized among 

 living rodents, eleven of these occur in Europe. 



KEY TO THE EUROPEAN FAMILIES OP HODENTIA. 



Upper inoisors 2-2, the enamel extending to pos- 

 terior surface; incisive foramina very large, 

 confluent ; bony palate much shorter than 

 mesopterygoid space ; fibula articulating with 

 calcaneum (Hares and Rabbits) Leporidm, p. 484. 



Upper incisors 1-1, the enamel confined to the front 

 surface ; incisive foramina moderate or small, 

 distinct ; bony palate longer than mesopterygoid 

 space ; fibula not articulating with calcaneum. 



* Members of the genera Mus and Oryctolagus are now artificially estab- 

 lished on many of the remote islands from which the order was naturally 

 absent. 



2 I 



